Wheel driving devices include an electric motor as a driving source, and a speed reducer through which the rotation of the motor is transmitted to a drive wheel after reducing its speed to drive the drive wheel. There are two types of such wheel driving devices, an in-wheel motor type and an on-board type.
FIG. 1(a) shows wheel driving devices A of the in-wheel motor type, which directly drive the respective rear wheels 1 as drive wheels. FIG. 1(b) shows wheel driving devices A of the on-board type, which drive the respective rear wheels 1 through drive shafts 3 connected through joints 2 to the output shafts of the speed reducers (not shown) in the respective wheel driving devices A.
Compact design is required for both wheel driving devices of the in-wheel motor type and those of the on-board type. Also, it is required that such wheel driving devices be capable of transmitting large torque to the rear wheels 1 to drive the rear wheels.
In order to answer these requirements, the below-identified Patent document 1 proposes a wheel driving device in which a cycloid speed reducer is used as the speed reducer for transmitting the rotation of the electric motor after reducing its speed.
A cycloid speed reducer includes an input shaft rotationally driven by the electric motor, and curved contoured plates as external gears rotatably mounted on respective eccentric shaft portions provided on the input shaft. This speed reducer further includes an outer pin retaining member as an internal gear fitted in the radially inner surface of a speed reducer casing supported by the vehicle body so as to be arranged coaxial with the input shaft. The outer pin retaining member supports outer pins as internal teeth which mesh with external teeth formed on the outer peripheries of the curved contoured plates and having an epitrochoidal profile. With this arrangement, when the input shaft is rotated, the curved contoured plates are eccentrically pivoted, so that the rotation of the curved contoured plates about their axes is transmitted from the output shaft, which is arranged coaxial with the input shaft.
In such a cycloid speed reducer, since the curved contoured plates as the external gears are eccentrically pivoted, it is necessary to rotationally fix the outer pin retaining member as the internal gear to the speed reducer casing. For this purpose, in the wheel driving device disclosed in Patent document 1, keys are used, each of which is fitted both in a key groove formed the radially inner surface of the speed reducer casing and in a key groove formed in the radially outer surface of the outer pin retaining member as the internal gear external to rotationally fix the outer pin retaining member.
In the wheel driving device disclosed in JP Patent Publication 2009-52630A, an elastic member is disposed between the speed reducer casing and the outer pin retaining member. When large radial loads or moment loads are applied to the outer pin retaining member while the vehicle is turning or during hard acceleration or deceleration, the elastic member is elastically deformed and thus absorbs such radial and/or moment loads. Thus, damage to the various parts of the wheel driving device including the curved contoured plates, outer pins as the internal teeth, and a motion converter mechanism for converting the eccentric pivoting motion of the curved contour plates to the rotary motion of the output shaft is prevented.
While the outer pin retaining member as the internal gear is kept afloat relative to the radially inner surface of the speed reducer casing with a gap left therebetween, by the elastic member, the internal gear is rotationally fixed by the keys. Thus, when the outer pin retaining member is vibrated by torque applied from the external gear, the vibration is transmitted to the speed reducer casing through the keys, which could generate noise.